Slick 60's

Been out of touch and even went to the old web site! Now that I found it again and have “Old Blue” back after some brake work, I have a couple of questions for someone in the group. I have received excellent advice from this group before! The old pickup is a work in progress and time and money allow.

I had a C6 installed in my 65 F250, Camper Special. Since the speedometer has been off about 10+% (At 55 MPH it Reads 65-70 MPH). As I remember it I put a 220 tooth sprocket/gear on the cable end, (the most you can get). The transmission place that installed it said that I would need to go up one tooth on an idler sprocket/gear inside the tranny. This doesn’t seem right to me? I would think you would go the other way? Can anyone tell me, first which way to go? UP or Down in teeth? I gave Certified Transmission (The installer) what I thought was the information needed to have it correct, by I guess not? I have a 390 CUI (69 T-Bird Engine), C6 Tranny and 16” wheels (750X16).

All help is appreciated, Thanks, Dale

Dale,
Engine & Tranny don't matter. Follow this through, numbers are example only. section width(235) X 2 X aspect ratio(75%) divide by 24.5 add wheel diameter(15) = tire diameter(29.39 inch) I used to have a conversion chart for the older measurements but seem to have lost it. You can measure from thhe ground up to a level placed across the top of the tire to obtain the rolling diameter, or a tire dealer may have the info. Then tire diameter(29.39) X pei(3.1416) = tire circumfrence(92.33). Then divide inchs per mile(63360) by the tire circumfrence(92.33) =tire revolution per mile(686). Then Drive gear count(6,7,8) X axle ratio(3.70) X tire revs per mile(686)+ X. Then divide X by 1000 = approx # of teeth to use on driven gear(17.7674). Then round off(17.7674 + 18). The driven gears are still available from dealers.
Fred

Dale,
Nope, just an old retired mechanic.
Fred

Quite the calculations, but basically you plug your numbers in the formula and away you go. I wish I could remember which way you go ..one tooth up or down for a fast speedo. I don't know if those little plastic gears go up in increments of 2 teeth or one, either.

If you want the speedometer to show more speed, use a gear with fewer teeth.

Think a multi-speed bicycle.

The front gear (at the pedals) is the driving gear, and it has the same function as the driving gear cut into the output shaft in the transmission.

The driven gear on the back wheel has the same function as the plastic gear on the end of the speedometer shaft.

If you want to the wheel (or speedometer) to turn more slowly, use the gear on the back wheel with more teeth - the larger one.

If you want the wheel (or speedometer) to go faster, use the gear on the back wheel with fewer teeth - the smaller one.

PS the gear you are changing out is at the end of the speedometer CABLE. not part of the transmission. you take the little retainer off at the transmission end and pull it carefully out. they are color coded as to how many teeth they have (green, grey white, etc). quite an easy fix

The driven gear(the one on the end of the cable) are available from your friendly Ford dealer from IIRC 14 to 21 or 22 tooth.
Fred

I didn't know there are 63,360 inches in a mile...good stuff to know for trivial pursuit game night.

Do the plastic gears on the end of the cables really go as high as 22? I just put an 11 tooth gear in my bumpside last week.

The range of the driven gears covers a lot of ratios and tire size combinations, then if by calculating, you come up with a higher or lower number than is available, you have to change the drive gear on the mainshaft.
Fred

TCI and some of the other trans companies have calculators in their webb sites. You need to know how many teeth on the trans drive gear, your rear end ratio and the height of your rear tire. Just plug in your numbers and the calcutator will figure it for you. 22 teeth normally is the most you can use although there is a 23 tooth gear out there but the cut of the teeth isnt just right and they dont last long.

Justa couple of quick notes of little or no importance. First off your engine can't be a 69 Tbird 390 because they did not make any. After mid 68 all Tbirds came with a 429 as the 390 was dropped (helpfull if you are trying to get parts because no books should show a 69 Tbird with a 390!). Second, the gear inside the trans does come in a couple of different versions and sometimes you do have to change it to get the speedo into range. Unfortunately there is a limit to the range of number of teeth as the gear is a finite size so there is more than one inner gear to allow for even more range. And finally the gears that go on the speedo cable end come in two flavors, left and right twist, be sure to get the correct ones if you are junkyard scavenging. Hawkrod

For the overthetop mathmatical approach to this, you should be aware that speedometers, at least the ones in the 50's - 70's are set up to have 1000 rotations per MILE. Of course, at 60 mph, that would also be 1000 rpm of the speedo cable, but that is immaterial.

By knowing some basic things about your setup, you follow all the rotations per mile through your drivetrain and know exactly where you are at.

For instance:


A tire that has a diameter of 30" has a circumference of 94.2" (2 x Pi x R) or (diameter x 3.14) Better yet is to find the manufacturers info about the "rolling circumference". Some manufactureres report rotations per mile. That's fine too.

To get rotations per mile, just divide 63360 (inches in a mile) by the circumference. For our example, comes to 672 rotations per mile.

Now, your rear diff ratio is reducing the speed of the axels from the speed of the driveshaft. So a 3.50 rear has the drive shaft spinning at 3.5 times the speed of the axel.

So multiply your rear axel speed times your rear diff speed, and you have the rotations of your driveshaft per mile.

That comes to 2353 (672 x 3.5). Now the speedo DRIVE gear on the tailshaft of the tranny is a spiral gear with 6-8 teeth (viewed from END, NOT the side, like petals on a flower). Lets assume that you have a 7 tooth spiral (most common)

That would mean that 16464 (7 x 2353) teeth pass by the speedometer driven gear each mile.

Now, the speedometer gears are available from perhaps 11 to 23 as mentioned, but you won't get good life from the extreems.

You want the speedo cable to rotate 1000 time per mile, remember? So if there are 16464 teeth going by the gear at the end of the speedo cable, you will need either a 16 or a 17 tooth driven gear. Both will be off a little, one way or the other.

The perfect driven gear would be 16.46 teeth (16464 / 1000) but of course, you can't have 16.46 teeth.

If you do not know your spiral (drive) gear teeth, you can calculate it, IF you know your tire size, rear axel ratio, and driven gear teeth #. You just need to drive 10 miles on a road that has mile markers, keep track of the odometer setting, and work back from there. If anyone is daunted by the reverese math, just post up the information and I will help you.

Thanks to all for the information. I read through my post again and first found that I fat fingered the number of teeth on the (driven) cable end. I bought it a the local ford dealer and I believe it is 22 teeth, and NOT 220 teeth. A lot of information, but I will print most, especially the formulas. My math skills need the step by step. I need to change the drive gear inside the C6

A also saw the post that the engine is not a 69 like I was told when I bought it from a co-worker in Laramie. Guess I will have to look at the block to see if it has more.

Another part that W forgot to say is the it is a Camper Special the rear end is geared low (4:10 ratio). Even though I am trying to get the speedometer right, eventially (When I can afford it), I want to change the rear end to between 3:50 and 3:80 so that while on the highway, the engine wont rap as high. Then I will go through it all over again.

Thanks again for all of the responses, Some good stuff! Dale